Logging exceptions in NestJS

Introduction

Logging exceptions helps you find and fix problems in your app quickly. It keeps a record of errors so you can understand what went wrong.

When you want to track errors happening in your NestJS app.

When you need to debug issues reported by users.

When you want to monitor app health and catch bugs early.

When you want to save error details for later analysis.

When you want to improve app reliability by fixing logged errors.

Syntax

NestJS

import { Logger } from '@nestjs/common';

try {
  // code that might throw
} catch (error) {
  Logger.error('Error message', error.stack);
}

Use Logger.error to log exceptions with a message and stack trace.

Import Logger from @nestjs/common to use NestJS built-in logging.

Examples

This logs a simple error with a message and the error stack.

NestJS

import { Logger } from '@nestjs/common';

try {
  throw new Error('Oops!');
} catch (error) {
  Logger.error('Caught an error', error.stack);
}

You can add a context string like 'MyApp' to identify where the error happened.

NestJS

import { Logger } from '@nestjs/common';

try {
  // some risky code
} catch (error) {
  Logger.error('Failed to process request', error.stack, 'MyApp');
}

Sample Program

This example shows how to catch an exception from a function and log it with a message, stack trace, and context.

NestJS

import { Logger } from '@nestjs/common';

function riskyOperation() {
  throw new Error('Something bad happened');
}

try {
  riskyOperation();
} catch (error) {
  Logger.error('Exception caught in riskyOperation', error.stack, 'AppModule');
}

Important Notes

Always log the stack trace to understand where the error happened.

Use context strings to make logs easier to search and filter.

Consider using custom exception filters in NestJS for centralized logging.

Summary

Logging exceptions helps track and fix errors in your app.

Use Logger.error with message, stack, and optional context.

Catch errors and log them to keep a clear error history.

Practice

1. What is the main purpose of logging exceptions in a NestJS application?

easy

A. To increase the app's memory usage

B. To speed up the application runtime

C. To track and fix errors by recording error details

D. To prevent users from accessing the app

Solution

1. Step 1: Understand the role of exception logging

   Logging exceptions records error details like messages and stack traces to help developers find and fix bugs.

2. Step 2: Identify the correct purpose

   Among the options, only tracking and fixing errors matches the purpose of logging exceptions.

3. Final Answer:

   To track and fix errors by recording error details -> Option C

4. Quick Check:

   Logging exceptions = track and fix errors [OK]

Hint: Logging exceptions helps find bugs by saving error info [OK]

Common Mistakes:

• Thinking logging improves app speed
• Confusing logging with security features
• Believing logging increases memory unnecessarily

2. Which of the following is the correct way to log an error with NestJS Logger inside a catch block?

easy

A. Logger.error(error.message, error.stack, 'MyContext')

B. Logger.log(error.message, 'MyContext')

C. Logger.warn(error.stack, 'MyContext')

D. Logger.debug(error.message, error.stack)

Solution

1. Step 1: Recall Logger.error method signature

   Logger.error takes three arguments: message, stack trace, and optional context string.

2. Step 2: Match the correct syntax

   Logger.error(error.message, error.stack, 'MyContext') correctly uses Logger.error with message, stack, and context. Others use wrong methods or missing parameters.

3. Final Answer:

   Logger.error(error.message, error.stack, 'MyContext') -> Option A

4. Quick Check:

   Logger.error(message, stack, context) = Logger.error(error.message, error.stack, 'MyContext') [OK]

Hint: Use Logger.error with message, stack, and context [OK]

Common Mistakes:

• Using Logger.log or Logger.warn instead of Logger.error
• Omitting the stack trace argument
• Passing arguments in wrong order

3. Given this NestJS code snippet inside a service method:

try {
  throw new Error('Failed operation');
} catch (error) {
  Logger.error(error.message, error.stack, 'MyService');
}

What will be logged?

medium

A. An info log with stack trace only

B. A warning log with message 'Failed operation' only

C. No log will be created because throw stops execution

D. An error log with message 'Failed operation' and stack trace under 'MyService' context

Solution

1. Step 1: Understand the try-catch behavior

   The error is thrown and immediately caught by catch block, so execution continues inside catch.

2. Step 2: Analyze Logger.error call

   Logger.error logs an error message with stack trace and context 'MyService'. This creates an error-level log entry.

3. Final Answer:

   An error log with message 'Failed operation' and stack trace under 'MyService' context -> Option D

4. Quick Check:

   Throw caught and logged as error with message and stack [OK]

Hint: Catch errors to log message and stack with Logger.error [OK]

Common Mistakes:

• Thinking throw stops all logging
• Confusing log levels (warn vs error)
• Ignoring the context parameter

4. Identify the error in this NestJS exception logging code:

try {
  // some code
} catch (error) {
  Logger.error(error.stack, error.message, 'AppModule');
}

medium

A. Arguments to Logger.error are reversed; message should come first

B. Logger.error cannot be used inside catch blocks

C. Missing await keyword before Logger.error

D. Logger.error requires only one argument

Solution

1. Step 1: Check Logger.error argument order

   Logger.error expects (message, stack, context). Here, stack and message are swapped.

2. Step 2: Confirm correct usage

   Swapping arguments causes wrong log output or errors. Correct order is message first, then stack trace.

3. Final Answer:

   Arguments to Logger.error are reversed; message should come first -> Option A

4. Quick Check:

   Logger.error(message, stack, context) order matters [OK]

Hint: Logger.error needs message first, then stack, then context [OK]

Common Mistakes:

• Swapping message and stack arguments
• Thinking Logger.error is async needing await
• Assuming Logger.error takes only one argument

5. You want to log exceptions in a NestJS global exception filter to capture all errors with context 'GlobalFilter'. Which code snippet correctly implements this?

hard

A. catch(exception: any) {
Logger.log(exception.message, 'GlobalFilter');
}

B. catch(exception: any) {
Logger.error(exception.message, exception.stack, 'GlobalFilter');
}

C. catch(exception: any) {
Logger.warn(exception.stack, 'GlobalFilter');
}

D. catch(exception: any) {
Logger.debug(exception.message, exception.stack);
}

Solution

1. Step 1: Understand global exception filter logging

   Global filters catch all exceptions; logging must use Logger.error with message, stack, and context.

2. Step 2: Evaluate each option

   catch(exception: any) { Logger.error(exception.message, exception.stack, 'GlobalFilter'); } correctly calls Logger.error with message, stack, and 'GlobalFilter' context. Others use wrong log levels or miss stack/context.

3. Final Answer:

   catch(exception: any) { Logger.error(exception.message, exception.stack, 'GlobalFilter'); } -> Option B

4. Quick Check:

   Global filter logs errors with Logger.error and context [OK]

Hint: Use Logger.error with message, stack, and context in global filters [OK]

Common Mistakes:

• Using Logger.log or Logger.warn instead of Logger.error
• Omitting stack trace or context
• Using Logger.debug which is too low level